152 Geological Society. 



the lower jaw of the Foetal Whale are fully developed in the Cachalot. 

 The upper rudimentary maxillary teeth which remain hidden in the 

 gum of the Sperm Whale are functionally developed in the Grampus; 

 and in like manner in the gigantic Dinotherium, discovered by Dr. 

 Kaup, is exhibited the full and functional development of the infe- 

 rior rudimental tusks of the Mastodon. 



The molar teeth of the Mastodons offer, Mr. Owen says, a beauti- 

 ful transitional modification connecting the lamellated structure of 

 the triturating molar with those having simply a transversely-ridged 

 grinding surface. The interval between the molar teeth of the 

 Elephant and those of the Tapir is too great to have allowed their 

 fundamental resemblance to have been detected in the existing 

 creation ; but a study of the extinct Pachyderms brings to light, 

 he says, a beautiful series of gradations leading through the ele- 

 phantoid Mastodon of Ava and the gigantic Mastodon of the 

 Missouri to the Dinotherium, which it may be remembered was the 

 gigantic Tapir of Cuvier. Moreover, he adds, the indication of the 

 singular armature of the loM'er jaw of the Dinothere might be most 

 closely discernible in that species of Mastodon which makes the 

 nearest approach to the Dinothere in the form of the grinding teeth. 



The report from which the above extracts have been taken had been 

 completed when Mr. Owen received a copy of the notice* of Dr. Hays's 

 description of Mr. Koch's collection. After an attentive perusal of 

 this document, in which the generic distinctness of the Tetracaulo- 

 don is maintained, Mr. Owen has been only more convinced of the 

 truth of his own theory ; he, however, in justice to Dr. Hays, gives 

 the arguments of that esteemed naturalist. Dr. Hays considers the 

 existence of a single tusk in the lower jaw to be only an accidental 

 occurrence, refemng, as examples of two tusks, to the specimen 

 described by Dr. Godman, and to that belonging to the Museum of 

 the University of Virginia. P^especting this statement, Mr. Owen 

 observes, that the jaw described by Dr. Godman is that of an im- 

 mature individual, retaining on the left side the first small molar, 

 and therefore affords no proof of the persistence of the two in- 

 ferior tusks in the adult animal, or evidence of the accidental na- 

 ture of the absence of the left tusk in the mature jaw. With regard 

 to the specimen in the cabinet of the University of Virginia, he 

 says, that if this belong to a mature animal it would be an unique 

 specimen, and might be paralleled with cases on record of two 

 projecting tusks in the male Narwhal, and considered by all na- 

 turalists to be accidental. Mr. Owen further calls attention to the 

 figure of the specimen in pi. 27. fig. 2. of the Transactions of the 

 American Philoso])hical Society (vol. iv.), where only the right tusk 

 is represented, the left being merely indicated by a dark spot of cor- 

 responding size, of the nature of which the text is silent. 



Respecting the symphysial portion of the jaw exhibiting the alve- 

 oli of two tusks, both much smaller than the alveolus of the right 

 tusk in the presumed male Mastodon's jaws of corresponding size, 

 and considered by Dr. Hays to constitute a distinct variety, if not a 

 new species of Tetracaulodon, Mr. Owen considers it to be the jaw of 

 * Proceedii)gs, Anierican Phil. Soc. October 1S41. 



